The reliable detection of stored binary data from any digital memory, while integral to successful application, can be a nontrivial task. Detection is performed by making binary decisions about the status of analog signal outputs from the memory in response to stored bits. It is not unusual that the memory signals available in the detection process are of low level and are subject to parametric variations and noise due to the physics of the memory technology and the manner of application. If the signals for a given memory type have significant variations across an ensemble of devices due to manufacturing differences or with changes in operating conditions, it may not be possible to recover data reliably with a nonadaptive detector. However, an adaptive detector can have an undesirable adaptation time requirement and can be complex and expensive.
In magnetic bubble memories, detection is particularly difficult due to the presence of a magnetic field which reorients in the plane of bubble movement in a well-understood manner. This "drive" field causes cyclical background noise in the detector. Further, changes in operating characteristics occur because of the material used for the layer in which bubbles move. Bubble memories in which bubbles move in response to such a drive field are called "field access" memories. Specifically, in some field access magnetic bubble memories (MBM), detection of stored binary data on a memory chip is accomplished by processing voltages that are measured across a matched pair of permalloy magnetoresistive sensor strips. Ideally, these sensors are subjected to the same magnetic, electrical, and environmental conditions with the exception that only one, an active sensor, is subjected to the (magnetic) H-fields of traversing bubble domains. Then the difference signal, typically between 1 and 10 millivolts, induced by a bubble across a sensor pair is detectable.
In practice, there are interference effects in the sensor outputs predominantly from control function drive signals, incomplete common mode rejection of H-field pickup, magnetoresistive switching noise, interference between bubble domains near the active sensor, and environmental effects such as chip temperature. Also, random thermal noise from the sensors, as well as thermal and shot noise from external detection electronics, are present in the observable differential sensor response.
In the drive field period of a field access MBM chip, there exists by design at least one time interval, defined here as the Detection Interval (D.I.), in which the sensor differential response is relatively free of inherent signal interference, and where a detectable difference should exist between responses for bubble and no-bubble cycles. For any MBM chip type, optimum determination of the D.I. relative to the drive field vector, and adherence to it when processing the sensor signals for data recovery, can be critical to successful memory performance.
Variations in the manufacture of a large ensemble of the same type of field access MBM chips with permalloy sensors can result in undesirable variations in signal shape, time of occurrence, and intensity of the differenced sensor outputs within any D.I. for both bubble and no-bubble responses. This can occur even when each chip of the ensemble is subjected to identical operating conditions such as a given D.I., field drive, chip temperature, recorded data pattern, etc. In addition, for any particular MBM chip of an ensemble, significant variations can occur in the differenced sensor outputs in any D.I. as a consequence of variations in field drive characteristics, bubble interaction at the sensor, and chip sensor temperature changes.
A complete parametric description of the family of sensor signals that are present for any MBM chip type has been intractable. Some general sensor signal variations that have been of major concern in previous nonadaptive MBM detectors are the sensor response negative temperature coefficient and the time shift of the dominant bubble response transition that occurs with drive field variations. In a detector with a fixed D.I. where the sensor response transition is compared to a fixed threshold, these two effects are intolerable.
Detectors can be designed that adapt to the variations in data 1 and 0 (Bubble-No Bubble) response characteristics when these variations preclude the use of a simple fixed threshold detector. For example, detector amplifier gain can be temperature-compensated to track the output signal variation due to permalloy magnetoresistive sensor sensitivity to temperature. However, adaptive detectors have the disadvantages of added circuit complexity and adaptation time.